Chill-cast ductile iron rolling mill rolls



United States Patent 3,273,998 CHILL-CAST DUCTgZELIlEON ROLLING MILL O L Roy J. Knoth, Westfieltl, N.J., Robert D. Schelleng, Suffern, N.Y., and Charles 12;. Isleib, Berkeley Heights, N.J., assignors to The International Nickel Company, New York, N.Y., a corporation of Delaware No Drawing. Filed May 13, 1964, Ser. No. 367,214

11 tilaims. (Cl. 75128) The present invention is directed to cast iron rolling mill rolls and, more particularly, to improved chill-cast rolling mill rolls made of ductile iron.

Ductile iron as a metallurgical material has been available to the art for over fifteen years and this new product of the cast iron foundry has received widespread acceptance in industry. It has been found that chill-cast ductile iron rolling mill rolls have provided adequate service in many applications involving the rolling of metals. The ductile irons which have been employed in this service have generally been chill cast and have had a pearlitic matrix with very substantial amounts of carbide in the chilled portions of the roll which have contributed a substantial degree of hardness to the roll. Such chill-cast ductile iron rolls have been useful but it has been found that in certain very severe rolling mill service such as in roughing mills, the chillcast ductile iron rolls heretofore available have not possessed the high level of strength and toughness required to render satisfactory service.

The problem of providing an improved rolling mill roll is a complicated one since each roll represents a very substantial weight of metal and each roll is an expensive casting. Furthermore, additional costs are encountered in shut-down time on the mill when rolls are changed. These cost factors make it a prohibitive matter to embark upon a testing program in which actual rolling mill rolls could be employed in practice to establish the relative degree of applicability of a particular roll for a particular service. These same cost factors have indicated that there was a substantial area for improvement of ductile iron rolling mill rolls.

An improved ductile iron rolling mill roll has now been discovered which provides an enhanced combination of properties, including increased hardness, strength and toughness as compared to presently available ductile iron rolling mill rolls.

It is an object of the present invention to provide special bainitic ductile iron compositions having special utility for service in cast rolling mill rolls.

It is a further object of the present invention to provide chill-cast ductile iron rolling mill rolls having high strength, hardness and toughness.

Another object of the invention is to provide chill-cast ductile iron rolling mill rolls having a bainitic structure and having properties which enable employment of the rolls in rolling mill roughing stands.

Other objects and advantages of the invention will become apparent from the following description.

Generally speaking, the present invention contemplates an improved chill-cast rolling mill roll having a bainitic structure in the chill portions thereof and having a high combination of strength, hardness and toughness made of a chill-cast ductile iron composition containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 0.6% manganese, about 3.5% to about 5.1% nickel, about 0.15% to about 0.9% chromium, about 0.4% to about 1% molybdenum, a small amount, e.g., about 0.02% or about 0.04%, up to about 0.1% magnesium, and with the balance essentially iron. Within the foregoing compositional ranges, particularly desired combinations of properties can be produced in chill-cast ductile iron rolls by relatively minor adjustments in composition. For example, when it is desired to emphasize strength in the chill-cast ductile iron roll, the nickel, chromium and molybdenum are maintained in the special ranges of about 4.6% to about 5%, about 0.5% to about 0.9%, and about 0.4% to about 0.8%, respectively. On the other hand, when it is desired to emphasize toughness in the chill-cast ductile iron roll provided in accordance with the invention, the nickel, chromium and molybdenum are maintained in the special ranges of about 3.5% to about 4%, about 0.15% to about 0.4%, and about 0.4% to about 0.6%, respectively. The actual amounts of alloying elements required in particular instances are also affected to some extent by the size of the roll casting involved since, as those skilled in the art will appreciate, the cooling rate of larger castings is slower than that of smaller castings. Thus, heavy rolls such as those having diameters of the order of 30 inches would require ductile iron compositions having the higher amounts of nickel and molybdenum and the lower amounts of silicon as compared to the foregoing special ranges which are designed particularly for rolls having diameters on the order of about 20 inches. Thus, a composition for maximum strength in a 30-inch diameter roll contains about 1.6% to about 2% silicon, about 4.7% to about 5.1% nickel, about 0.5 to about 0.9% chromium, and about 0.5 to about 1% molybdenum. On the other hand, a composition for maximum toughness in a 30-inch diameter roll contains about 1.6% to about 2% silicon, about 3.8% to about 4.2% nickel, about 0.15% to about 0.4% chromium, and about 0.5 to about 0.8% molybdenum.

Each of the elements in the chill-cast ductile iron roll composition is maintained within the ranges given to provide the improved results from the standpoints of hardness, toughness, strength and wear resistance characterizing the chill-cast ductile iron rolls provided in accordance with the invention. Thus, silicon and chromium are maintained within the ranges given since they importantly control hardness and wear resistance of the chill-cast roll through control of the amount of carbide present in the chill portions of the roll. The amount of carbide in the microstructure may be in the range of about 2% to about 40%, e.g., about 5% to about 30%, on the outer or chill portions of the roll and, most advantageously, is about 10% to about 20% in order to obtain the best combinations of properties, including wear resistance, hardness, strength and toughness. Excessive carbide results in unduly increased hardness and loss of strength and can occur as a result of chromium contents at excessive levels or because of an insufiicient silicon content. In addition, chromium at levels above about 0.9% embrittles the matrix and causes low strength and toughness. Nickel and molybdenum in the chill-cast ductile iron roll govern the matrix structure. Accordingly, both nickel and molybdenum must be maintained within the aforementioned ranges to prevent the formation of any substantial amount of pearlite in the matrix. On the other hand, molybdenum must not exceed the maximum amount set forth hereinbefore as otherwise too great a formation of massive carbide and loss of strength results. When molybdenum exceeds about 1% and nickel exceeds about 5.1% an unduly increased hardenability with the production of martensite and accompanying low strength results. Manganese is a carbide former and also mildly increases hardenability. The presence of a moderate amount is useful for these purposes but when more than 1% is present the amount of carbide increases causing embrittlement and loss of strength. High manganese contents also may cause retention of untransformed aus tenite and loss of hardness. Magnesium in the ductile iron roll composition controls graphite in the roll casting to the spheroidal form and contributes improved toughness to the roll structure.

When all the allowing ingredients are controlled as set forth hereinbefore, the structure in the outer portions of the chill-cast ductile iron roll is bainitic and a high combination of hardness, strength and toughness is achieved in the roll. The special ductile iron roll castings provided in accordance with the invention may also contain usual impurities such as copper less than about 0.5%, vanadium and tungsten less than about 0.2% each, phosphorus less than about 0.12%, sulfur not more than about 0.01%, aluminum and titanium less than about 0.05% each, and lead, antimony, and bismuth less than about 0.002% each. About 0.005% rare earths, e.g., cerium, etc., is beneficial to control graphite shape.

Since, as noted hereinbefore, roll castings are large expensive castings, it is impractical from the cost standpoint to enter into a testing program involving sectioning and use testing of actual rolls. Accordingly, a special test casting was devised which provided a substantial sized casting for test purposes and which had a cooling rate comparable to that encountered in a chilled iron roll about 20 inches in diameter. The casting was a five inch by five inch by ten inch block poured horizontally in dry sand with one end being cast against a five inch 'by five inch by five inch carbon-coated iron chiller. Again, the variety of microstructures encountered in a chilled casting due to the great difference in cooling rate between the metal adjacent the chiller and the metal in roll sections at points progressively removed from the chiller presented problems in obtaining comparable data even in the test castings. This factor indicated that standard tensile tests and impact tests would not represent the properties at any particular portion of the casting but would involve an averaging of the properties of the casting across the section represented by the test specimen. Accordingly, the static bend test was employed to provide an indication of the strength and toughness of the roll irons investigated. These roll irons were, relatively speaking, quite hard and nonductile. The bend specimen employed was five inches long, one-quarter inch high and three-quarter inch wide. In testing, the specimen was placed across supports spaced four inches between centers .and a load was applied at the mid-point between the supports to effect deflection and ultimate fracture of the test specimen. The ultimate bend strength was calculated from the standard elastic flexure formula for beams having a concentrated load at the midpoint. Toughness was evaluated by integrating the area under the load-deflection curve. In each case, the bend specimens were finish-ground and the long edges were beveled to minimize corner effects. Specimens were cut at various distances up to five inches from the chilled face of the casting and parallel thereto. In this manner, it was possible to provide a bend test specimen having a uniform structure on the tension face. In bend testing with materials of the nature involved, the calculated ultimate strength is approximately twice the ultimate strength measured in the conventional tensile test.

In order to give those skilled in the art a better understanding of the invention and/ or a better appreciation of the advantages of the invention, the following illustrative examples are given:

Ductile iron castings produced in the aforedescribed manner and having the compositions set forth in the following Table I were produced by melting pig iron, low carbon steel, electrolytic nickel and ferro alloys in an air induction furnace. After melting, the charge in each case was heated to about 2800 F. to dissolve all the alloying ingredients. The melts in each case were treated at 2700 F. with a 0.8% addition of a nickel-magnesium alloy containing about magnesium after which the melts in each case were reladled and inoculated with a 0.5% addition of ferrosilicon containing 85% silicon. The inoculated metal was then cast to produce the test block described hereinbefore. The castings in each case were poured at a temperature of about 2500 F.

TABLE I Per- Per- Per- Per- Per- Per- Per- Per- Per- Alloy N0. cent cent cent cent cent cent cent cent cent C Si Mn Ni Cr Mo S P Mg Specimens from the castings were subjected to the bend test described hereinbefore and in each instance the hardness of each bend specimen was obtained using the Scleroscope hardness tester. The results of the tests are set :forth in the following Table II:

1 Derived by integration of area under load-deflection curve.

In addition, microsections of each of the castings 'were micrographically examined and it was observed in each instance that the structure was acicular (bainitic).

For comparison purposes, two pearlitic nickel-chrom ium-molybdenum alloyed ductile irons of the composition currently employed in rolling mill roll service were prepared in accordance with the procedure described hereinbefore and castings produced from these irons were subjected to testing in the same manner. The compositions of the two pearlitic irons are set forth in the following Table III and the test results obtained on these irons are set forth in the following Table IV:

TABLE III Per- Per- Per- Per- Per- Per- Per- Per- Per- Alloy No. cent cent cent cent cent cent cent cent cent 0 Si Mn Ni Cr Mo S 1 Mg TABLE IV Dis- Ultimate Total Modulus Energy Per- Allo tance Bend Defiecof Elasto Hardcent No. from Strength, tion, ticity, Fracness, Car- Clnll, p.s.i. in. 10 ,p.s.i. ture, Sc. bide in. ft. lb.

1 Derived by integration of area under load-deflection curve.

By comparison of results obtained in castings produced in accordance with the invention to prior art irons, it is to be seen that the special cast irons produced in accordance with the invention are at the same time harder, stronger and tougher than the prior art pearlitic irons. These greatly improved combinations of properties in the rolling mill rolls prorvided in accordance with the invention provide special improved utility therefor in severe rolling mill service.

The improved combinations of properties attained in ductile iron rolls produced in accordance with the invention makes possible longer roll life, reduces the danger of breakage, and enables the use of ductile iron rolls in heavy duty rolling mill stands such as roughing mill stands. The special ductile iron castings produced in accordance With the invention are also useful for wear resistance applications such as secondary crusher liners, large ball-mill crusher liners, and the like.

As noted hereinbefore in connection With the examples, it is advantageous to employ a graphitizing inoculation to produce ductile iron castings in accordance with the invention even though chill-type castings are produced. Graphitizin-g inoculation in accordance with the invention provides the advantages that the desired spheroidal graphite structure and the desired carbide structure are reproducibly obtained. Castings might be made without inoculation in special circumstances but obtaining of the above desired structures could not be insured.

The special ductile iron compositions provided in accordance with the invention are generally contemplated for use in the production of unitary rolls but may also be employed for the manufacture of the outer portions of composite rolls such as double-poured rolls and mechanically produced composite rolls wherein a sleeve of one material is placed about a core of another material.

Although the present invention has been described in conjunction With preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the invention and appended claims.

We claim:

1. A bainitic ductile iron chill casting containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 0.6% manganese, about 3.5% to about 5.1% nickel, about 0.15% to about 0.9% chromium, about 0.4% to about 1% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

2. A bainitic ductile iron chill casting containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 0.6% manganese, about 4.6% to about 5% nickel, about 0.5% to about 0.9% chromium, about 0.4% to about 0.8% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite to the spheroidal form, and the balance essentially iron.

3. A bainitic ductile iron chill casting containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 0.6% manganese, about 3.5 to about 4% nickel, about 0.15 to about 0.4% chromium, about 0.4% to about 0.6% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal tform, and the balance essentially iron.

4. A bainitic ductile iron chill casting containing about 3% to about 3.3% carbon, about 1.6% to about 2% silicon, about 0.3% to about 0.6% manganese, about 4.7% to about 5.1% nickel, about 0.5% to about 0.9% chromium, about 0.5% to about 1% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

5. A bainitic ductile iron chill casting containing about 3% to about 3.3% carbon, about 1.6% to about 2% silicon, about 0.3% to about 0.6% manganese, about 3.8% to about 4.2% nickel, about 0.15% to about 0.4% chromium, about 0.5% to about 0.8% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

6. A chill-cast ductile iron roll having at least the outer portions thereof made of an alloy containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 0.6% manganese, about 3.5% to about 5.1% nickel, about 0.15 to about 0.9% chromium, about 0.4% to about 1% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

7. A chill-cast ductile iron roll having at least the outer portions thereof made of an alloy containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 013% to about 0.6% manganese, about 4.6% to about 5% nickel, about 0.5% to about 0.9% chromium, about 0.4% to about 0.8% molybdenum, a small amount up to about 0 .1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

8. A chill-cast ductile iron roll having at least the outer portions thereof made of an alloy containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 0.6% manganese, about 3.5% to about 4% nickel, about 0.15 to about 0.4% chromium, about 0.4% to about 0.6% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite there-in to the spheroidal form, and the balance essentially iron.

9. A chill-cast ductile iron roll having at least the outer portions thereof made of an alloy containing about 3% to about 3.3% carbon, about 1.6% to about 2% silicon, about 0.3% to about 0.6% manganese, about 4.7% to about 5.1% nickel, about 0.5 to about 0.9% chromium, about 0.5 to about 1% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balthe balance essentially iron.

10. A chill-cast ductile iron roll having at least the outer portions thereof made of an alloy containing about 3% to about 3.3% carbon, about 1.6% to about 2% silicon, about 0.3% to about 0.6% manganese, about 3.8% to about 4.2% nickel, about 0.15% to about 0.4% chromium, about 0.5% to about 0.8% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

11. An alloy ductile iron having a high combination of Wear resistance, hardness, strength and toughness containing about 3% to about 3.3% carbon, about 1.6% to about 2.2% silicon, about 0.3% to about 1% manganese, about 3.5% to about 5.1% nickel, about 0.15 to about 0.9% chromium, about 0.4% to about 1% molybdenum, a small amount up to about 0.1% magnesium to control the occurrence of graphite therein to the spheroidal form, and the balance essentially iron.

References Cited by the Examiner UNITED STATES PATENTS 1,910,034 5/1933 Mitchell -1Q8 X 1,948,246 2/1934 Seaman 75128 X 2,771,358 1 1/ 1956 Spear 75128 DAVID L. =RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

11. AN ALLOY DUCTILE IRON HAVING A HIGH COMBINATION OF WEAR RESISTANCE, HARDNESS, STRENGTH AND TOUGHNESS CONTAINING ABOUT 3% TO ABOUT 3.3% CARBON ABOUT 1.6% TO ABOUT 2.2% SILICON, ABOUT 0.3% TO ABOUT 1% MANGANESE, ABOUT 3.5% TO ABOUT 5.1% NICKEL, ABOUT 1% MOLYBDENUM, A SMALL CHROMIUM, ABOUT 0.4% TO ABOUT 1% MOLYBDENUM, A SMALL AMOUNT UP TO ABOUT 0.1% MAGNESIUM TO CONTROL THE OCCURRENCE OF GRAPHITE THEREIN TO THE SPHEROIDAL FORM, AND THE BALANCE ESSENTIALLY IRON. 